Ethylene copolymers have heretofore been molded by various molding methods, and used in many fields. The requirement for the characteristics of the ethylene copolymers differs depending on the molding methods and uses. For example, when an inflation film is molded at a high speed, it is necessary to select an ethylene copolymer having a high melt tension compared with its molecular weight in order to stably conduct high speed molding without fluctuation or tearing of bubbles. An ethylene copolymer is required to have similar characteristics in order to prevent sag or tearing in blow molding, or to suppress width shortage to the minimum range in T-die molding.
Further, in extrusion molding, it is important to have small stress under high shearing during extrusion in order to improve quality of molded article and reduce electric power consumption at molding.
By the way, Japanese Patent L-O-P Nos. 90810/1981 and 106806/1985 propose a method for improving moldability by improving the melt tension and blow ratio (die/swell ratio) of ethylene polymers obtained by using Ziegler type catalysts, especially a titanium type catalyst.
The ethylene polymers obtained by using a titanium catalyst, however, especially the low density ethylene polymers generally have problems such as their broad composition distribution and stickiness of their molded articles such as films.
Of the ethylene polymers prepared by using the Ziegler type catalysts, those obtained by using chromium type catalysts are relatively excellent in melt tension but have a defect of poor heat stability. This is thought to be caused by that the chain terminals of the ethylene polymers prepared by using the chromium type catalysts tend to become unsaturated bonds.
It is known that the ethylene polymers obtained by using a metallocene catalyst from among the Ziegler type catalysts have merits such as a narrow composition distribution and a low stickiness of their molded articles such as films. However, it is described in, for example Japanese Patent L-O-P. No. 35007/1985, that an ethylene polymer obtained by using a zirconocene compound formed from a cyclopentadienyl derivative contains one terminal unsaturated bond per molecule, and hence this ethylene polymer is presumably poor in heat stability similarly to the above-mentioned ethylene polymer obtained by using the chromium type catalyst. Further, because of its narrow composition distribution, this ethylene polymer might show poor flowability during the extrusion molding.
Accordingly, the advent of ethylene polymers having a small stress under the high-shear region, a good heat stability, a high mechanical strength and a narrow composition distribution will be of great industrial value.
The present researchers have earnestly studied in the light of the circumstances as described above. As a result, they have found that the ethylene/.alpha.-olefin copolymer obtained by copolymerizing ethylene with an .alpha.-olefin of 3 to 20 carbon atoms in the presence of a specific catalyst for olefin polymerization comprising
(a) a compound of a transition metal in Group IVB of the periodic table which has a bidentate ligand formed by bonding two groups selected from specific indenyl or substituted indenyl groups through a lower alkylene group, or a compound of a transition metal in Group IVB of the periodic table which has a ligand having specific cyclopentadienyl skeleton, and
(b) an organoaluminum oxy-compound is excellent in melt tension and heat stability and has a narrow composition distribution. However, the ethylene/.alpha.-olefin copolymer as mentioned above is not always well-balanced between the melt tension and the flowability, so that a problem sometimes occurs when the copolymer is subjected to extrusion molding to form a film.
The present inventors have further studied and found that the ethylene copolymer composition comprising two kinds of ethylene/.alpha.-olefin copolymer different in both a density and an MFR from each other which are obtained by using the above-mentioned catalyst is excellent in heat stability, melt tension and flowability (FI) under the high-shear region, and films obtained from these compositions are excellent in transparency, mechanical strength and blocking resistance.